Fuel cells have recently attracted a great deal of attention, and several types of fuel cells have been developed. Among others, polymer electrolyte fuel cells are expected to be put into practical use as a power source for low-pollution electric vehicles.
FIG. 1 is an exploded view showing an example of the basic structure of a polymer electrolyte fuel cell. As illustrated therein, a single fuel cell of this type has a layered structure having a membrane of a polymer electrolyte 10, a membrane of a fuel electrode (anode) 12 disposed on one surface of the electrolyte membrane and a membrane of an oxidizer (air) electrode 14 disposed on the other surface thereof. A separator 16 is provided on each surface of the layered structure. Several tens to several hundreds of such single cells are stacked in a fuel cell for actual use. Although not shown, each surface of the polymer electrolyte membrane is coated with a catalyst comprising fine carbon particles and ultrafine noble metal particles.
A separator for a fuel cell which is also referred to as a bipolar plate serves to achieve electric connection between adjacent cells and provide means for feeding a fuel gas and an oxidizer gas, and it also functions as a plate for isolating these gases. To this end, as shown in the figure, a number of grooves or channels 16a and 16b are formed in a separator so as to constitute flow paths for gases being supplied and discharged.
The materials used for separators are roughly divided into non-metallic materials such as graphite and carbon and metallic materials. From the standpoint of costs and size reduction, there is a desire to change from non-metallic materials to metallic materials. Metallic materials which can conceivably meet the requirement for corrosion resistance include stainless steels, aluminum, nickel-iron alloys, titanium, and the like. Of these, stainless steels have been investigated most actively.
However, stainless steels have a very high contact resistance due to the presence of a passive film (passive state oxide film) on the surface. For example, a typical commercially available stainless steel sheet has a contact resistance as high as 100 mΩ·cm2 or higher, which causes the electric connection between cells to be poor. Furthermore, the corrosion resistance of stainless steels is not sufficient in the environments encountered in fuel cells so that dissolution of metal takes place to form metal ions, which deteriorate the performance of the catalyst supported on an electrolyte membrane. In addition, the dissolved metal ions result in the formation of corrosion products such as Cr—OH and Fe—OH on the surface of a separator, thereby increasing the contact resistance of the separator. Therefore, with a separator made of stainless steel, a noble metal plated layer such as a gold plated layer is interposed between the separator 16 and each of the electrode membranes 12, 14 so as to extend over the contact portions therebetween and thus ensure the necessary electrical continuity and corrosion resistance. However, this technique requires very high costs, which limit the practical use of the cell.
A fuel cell is heavy, since it is a stack of a large number of single cells like that described above, so weight saving is strongly desired for fuel cells for use in vehicles such as automobiles. For this reason, a titanium-based material which can be lighter in weight and higher in corrosion resistance compared to stainless steel sheets has attracted attention as a material for separators of fuel cells.
The term “titanium-based material” used herein is intended to include both titanium metal and titanium alloys.
It is well known that a titanium-based material is lighter and has improved corrosion resistance compared to other metallic materials, but a passive film having a high electric resistivity exists on the surface thereof, as is the case with stainless steel. In general, a passive film is necessary for a metallic material to have corrosion resistance, and the thicker the passive film, the better the corrosion resistance. However, when the metallic material is used as a separator, the passive film results in an increased contact resistance. In the case of a titanium-based material, since plating can not readily be applied, plating with a noble metal cannot be employed as a means for decreasing the contact resistance.
It is proposed in JP 2001-357862-A1 that electrically conductive hard particles be embedded in the surface of a titanium-based material for fuel cell separators in such a manner that they are exposed on the surface of the material, thereby making it possible to utilize the particles as conducting paths and decrease the surface resistance. The conductive hard particles are formed from metal carbides of the M23C6, M4C, or MC type (M: metal), and they are embedded in the surface of the titanium-based material by shot impinging. However, shot impinging required a complicated operation leading to an increase in costs. Moreover, the hard particles which are electrically conductive exist only in a surface region of the titanium-based material. Since the electrical conductivity of a titanium-based material is lower than that of stainless steel, the presence of the conductive particles only in the surface region cannot decrease the contact resistance adequately, thereby increasing the Joule heat generated with passage of electric current and hence the costs required for cooling.